Just: spell out that a multi-line separator is multiple shell lines

"Has to be a comment itself" reads as "has to start with `#`", and a value
whose first line does while its later lines do not is not a comment -- every
line after the first is its own shell line and runs.

No mechanism for it. Flattening newlines would leave a single-line value
that is not a comment still running while a multi-line one is defused: two
spellings of the same mistake with opposite outcomes and no principle
telling them apart. The obligation is on the value, so say what it is.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: redsun82

misc/just/README.md

@@ -162,7 +162,7 @@ for itself, and the count is one per `mod` reached, however deeply nested, plus
 the file itself. That is cheap, and modules agree anyway since they share a terminal,
 but it is worth knowing before counting measurements. Presetting `JUST_CMD_RULE` skips
 all of them, and is also how to fix the width in CI or in a recording — it is the whole
-line, so set it to something the shell will treat as a comment.
+separator and ends up in a shell script, so every line of it has to be a comment.
 
 With no terminal to ask — a pipe, a log, a shell without `stty` — it falls back to a
 fixed 57 columns, so logs and CI output are the same width every time. That is one

misc/just/defs.just

@@ -23,8 +23,9 @@ error := f'{{ style("error") }}error{{ NORMAL }}: '
 # With no terminal the value is non-numeric and this falls back to a fixed 57 columns.
 # That is a branch, not a platform test: just runs `sh` everywhere.
 #
-# `JUST_CMD_RULE` is the whole line and lands in a shell script, so a value set by hand
-# has to be a comment itself.
+# `JUST_CMD_RULE` is the whole separator and lands in a shell script, so a hand-set
+# value has to be a comment -- every line of it, if it has more than one. Nothing checks
+# that; a value that is not one gets run.
 #
 # Exported and preferred over measuring: `shell()` runs on every parse, so a forwarded
 # verb would otherwise re-measure in every child it spawns, and `if` is lazy in its